Polarized neutron imaging and three-dimensional calculation of magnetic flux trapping in bulk of superconductors

Abstract

Polarized neutron radiography was used to study the 3D magnetic flux distribution inside of single crystal and polycrystalline Pb cylinders with large (cm 3 ) volume and virtually zero demagnetization.Experiments with single crystals being in the Meissner phase (T<Tc) showed the expected expulsion of magnetic field. 99.9999 weight % pure polycrystalline samples were exposed to the same homogeneous magnetic field (6.4mT) and only a portion of the applied field was expelled. The trapped field in the sample (T < T c , Bext = 0 T), showed a nearly Gaussian spatial distribution, centered on the cylinder axis and decreasing towards the surface of the cylinder. In the direction along the cylinder axis the trapped flux was nearly constant. The expelled field outside of the samples followed 1/R dependence. These measurements provided a novel and detailed picture of macroscopic superconducting samples, confirming the existence of both uniform bulk Meissner expulsion in single crystals and bulk flux trapping with nearly Bean-model profiles due to flux pinning in polycrystalline samples.

Equilibrium intermediate-state patterns in a type-I superconducting slab in an arbitrarily oriented applied magnetic field

“…Experimentally, in addition to various macroscopic and indirect techniques, magnetic flux structures in type-I superconductors were visualized on the sample surface by using a Bi wire as a magnetoresistive probe, 30,31 by decoration with small diamagnetic 32 or ferromagnetic [33][34][35] particles, by the electron mirror technique, 36 by using the magneto-optical Faraday effect, 10,14,16,26,[37][38][39] by using miniature scanning Hall probes 40,41 and, in the bulk, by using polarized neutron reflectometry 42,43 and muon spin rotation. 44 Unlike type-II superconductors where the magnetic field can appear only in the form of single-flux-quantum Abrikosov vortices, 45 the mix of normal and superconducting domains in the intermediate state of type-I superconductors exhibits diverse geometric patterns, and their shape and distribution depend sensitively on many factors, including chemical, mechanical and geometrical parameters of the studied samples, 9,23,26,46 history of how magnetic fields and temperature were varied, direction of the magnetic field with respect to the sample, and dynamical perturbations such as electric currents or ac fields.…”

Neutron Dark-Field Imaging of the Domain Distribution in the Intermediate State of Lead

“…However, typical IS domain sizes of several µm are not accessible by standard scattering techniques such as small-angle neutron scattering (SANS). Moreover, polarized neutron radiography [14], which has been used for the imaging of trapped flux in SCs, is lacking resolution for a detailed study of the IS structure [15].…”

Improving polarized neutron imaging for visualization of the Meissner effect in superconductors

“…The neutron's non-destructive nature along with its deep penetration makes PNI a unique technique for studying magnetic fields inside samples. 5,6 This technique has been implemented at several neutron imaging beamlines. 7,8 The behavior of the neutron polarization in a magnetic field is described by the Bloch equation,…”